Transcript The Quadrupole Cusp: A Universal Accelerator, or From Radiation

The Quadrupole Cusp: A
Universal Accelerator,
or
From Radiation Belts to
Cosmic Rays
Robert Sheldon
NSSTC Seminar, March 19, 2004
Outline
• Quadrupoles
– Plasma Thermodynamics
– Acceleration Mechanisms and Entropy
•
shocks, waves, reconnection and dipole
• Why a trap improves acceleration efficiency
– How a Quadrupole Traps & Accelerates
– Signatures of Quadrupole Acceleration
• Application 1: Outer Radiation Belt Electrons
• Application 2: Galactic Cosmic Rays
Introduction
• Why is studying acceleration mechanisms
important?
– Fundamental physics (e.g. Nobel Prizes)
– Usefulness in medicine, science, technology
• Radiation therapy, advanced light sources,
microwave ovens etc.
– Prediction of harmful radiation to satellites and
humans in space.
• NASA goals, even Moon missions!
– Plain fun.
Two “unsolved” physics problems
• Source of the highly variable, outer radiation
belt electrons
• Source of Galactic cosmic rays.
McIlwain, 1966
Galactic Cosmic Ray Spectrum
Plasma Thermodynamics
• A neglected field—so this discussion is very
qualitative. (Please give me references!)
• In ideal gases the Maxwell equilibrium is extremely
well maintained by collisions
• In plasmas, turbulence takes the place of collisions.
But the long-range Coulomb interaction means that
equilibrium is often attained very slowly.
• Therefore one must get accustomed to nonMaxwellian distributions, and non-standard
measures of temperature and entropy. E.g. kappas
What is Acceleration?
• Entropy is often defined as S = Q/T. Thus highly
energetic particles have low entropy. Often they are
“bump-on-a-tail” distributions, or “power law”
greater than Maxwellian. Such low-entropy
conditions cannot happen spontaneously, so some
other process must increase in entropy. Separating
the two mechanisms, we have a classic heat-engine.
Note: the entropy INCREASE of
heat flow into the cold reservoir, is
counterbalanced by the DECREASE
of entropy into the Work.
Efficiency of Acceleration
• The efficiency of a heat engine is easy to describe:
e=W/Q1. The work produced for some amount of
heat. We can also use a slightly modified definition
by dividing by temperature, T: e=Sw/SQ
• We can now use these definitions to categorize the
various acceleration mechanisms that have been
proposed for radiation belts and cosmic rays.
–
–
–
–
Waves
Reconnection
Fermi I and II
Shocks
Very Low
Low
Medium
Low
< 10-4
< 10-3
< 10-1
< 10-3
Why does a Trap help?
• Single-stage acceleration mechanisms operate very
far from equilibrium. They therefore have a huge
difference in entropy between W and Q, and are
therefore inherently inefficient. Likewise
environmental conditions are far from equilibrium,
and are thus inherently unlikely. The product of
these two situations = low density of accelerated
particles.
Energy Source * Efficiency * Probability = Power
• A trap allows small impulses to be cumulative. The
pulses are close to equilibrium and are also likely.
Thus a trap = higher density of accelerated particles.
The Fermi-Trap Accelerator
Waves convecting with
the solar wind, compress
trapped ions between the
local |B| enhancement
and the planetary bow
shock, resulting in 1-D
compression, or E//
enhancement.
The Dipole Trap
• The dipole trap has a positive B-gradient that causes
particles to trap, by B-drift in the equatorial plane.
There are 3 symmetries to the Dipole
each of which has its own “constant of
the motion” (Hamiltonian is periodic).
1)Gyromotion around B-field
Magnetic moment, “”;
2) Reflection symmetry about
equator Bounce invariant “J”;
3) Cylindrical symmetry about z-axis
Drift invariant “L”
The Quadrupole Trap
• A quadrupole is simply the sum of two dipoles.
– Dipole moving through a magnetized plasma,
heliosphere, magnetosphere, galactosphere
– A binary system of magnetized objects –binary stars
– A distributed current system-Earth’s core, supernovae
• Quadrupoles have “null-points” which stably trap
charged particles (eg. Paul trap used in atomic
physics)
• Maxwell showed that a perfect conducting plane
would “reflect” a dipole & form a quadrupole
Maxwell 1880
Chapman 1930
Tsyganenko Cusps
Why is the Dipole a better trap
than Accelerator?
• Three “adiabatic invariants” (, J, L)
w/characteristic times of 1ms, 1s, 1000s
• KAM theorem: the 3 adiabatic invariants are
separated by factors of 1000 in time, and therefore
do not “mix”. So dynamical evolution, or
diffusive/stochastic “acceleration” is not fast.
• Arnol’d diffusion, or chaotic motion in the Poincaré
section, can lead to rapid diffusion if these “basins”
are connected to each other--“Arnol’d web”
Kolmogorov, Arnol’d, Moser Theorem
Earth orbit as
Perturbed by
Jupiter.
Poincaré slice
x vs. vX taken
along the E-J
line.
Earth orbit if
Jupiter were
50k Earth
masses.
Comparison of Dipole & Quadrupole
• Rim fed, center exit
• Exit blocked by magnet
• Compressions mostly affect
the low-energy part
• High-E particles are better
trapped than Low-E
• Adiabatic invariant times
separated by factor ~1000
• Scattering (diffusion) out of
the trap is difficult and deenergizing / deaccelerating.
• Center fed, rim exit
• Exit accessible
• Compressions affect whole
distribution
• High-E particles less
trapped than Low-E
• Adiabatic invariant times
separated by factor ~10
• Scattering out of trap is
easy and may also
energize/ accelerate
Quadrupole Trap in
the Laboratory
(Two 1-T magnets, -400V,
50mTorr)
Particle Tracing in T96 cusp, untrapped
Periodic, well trapped
Chaotic, barely trapped
Chaotic, nearly trapped
e- Trapping Regions of T96 Cusp
H+ Trapping in T96 Cusp
Poincaré Plots?
• Now that we know there are trapping regions of the
cusp, e.g. periodic orbits, we can display those orbits
with a Poincaré section. Then we can potentially
demonstrate the invariant tori and chaotic orbits.
• Unfortunately, I only realized the advantage of this
approach after plotting 4000 trajectories T<drifttime and visually classifying them as “untrapped”,
“trapped periodic”, and “trapped chaotic”. (Using 4
CPU weeks on dual AMD 1.8GHz)
• So the Poincaré plot (many drift orbits) will have to
wait for CPU time.
How does the Quadrupole Accelerate?
• Mathematically, we say that the basins of chaos are
interconnected, not constrained by invariant tori.
They are connected in an Arnol’d web that allows
rapid stochastic acceleration.
• More prosaically, compressions adiabatically
increase the particle energy, while the center of the
trap acts as a “mixmaster” for redistributing that
energy into other invariants. Consequently particles
diffuse in energy very rapidly.
• A comparison to Fermi I,II is highly instructive for
understanding the mechanism.
Acceleration via Random Impulses
• Fermi-I acceleration can be thought of as a 1-D
compression of a magnetized plasma. It heats only
in the E// direction. Eventually the pitchangle gets
too small to be reflected by the upstream waves, and
it escapes. If pitchangle diffusion occurs, a particle
may convert E//E_perp, and continue to gain
energy.
• The waves, or compressive impulses need not be
coherent or particularly large, especially if scattering
is occurring. Thus the entropy of the energy source
is relatively high, the probability is high = high
efficiency acceleration. Observations support this.
Cusp
Orientation
When the cusp points
toward the sun, it
“opens”, when it
points away, it
“closes”. Likewise,
SW pressure pulses
will shrink the cusp as
a whole, e.g. radially
compressing it.
Fermi I,II
vs
•
• 1-D compression, E//
• Upstream Alfven waves
•
impinging on barrier
• Scattering inside trap due to •
waves
• Max Energy due to scale
•
size of barrier (curvature)
becoming ~ gyroradius
• Acceleration time
•
exponentially increasing
• Critically depends on angle •
of SW B-field with shock
Alfven I,II
2-D compression, E_perp
Compressional waves
impinging on cusp
Scattering inside trap due to
quadrupole null point
Max energy due to
gyroradius larger than cusp
radius (rigidity).
Acceleration time
exponentially increasing
Critically depends on cusp
angle with SW
Signatures of Cusp Acceleration
• Gyroradius effects: r = (mE)1/2/qB. For a given
topology, lighter masses will have higher cutoff
energies.
• Energy increase is exponentially increasing function
of time. (like Fermi)
• Scale size effects: the larger the cusp, and the larger
B-field, the higher the cutoff E.
• Output spectra have power law tails (not bump-ontail nor exponential maxwellians.)
• Given continuous input, output is also continuous.
Problem #1: Source of ORBE
Prediction Problems
• The hazards of MeV electrons are greatest in the
outer radiation belts (L~3-5), though they still exist
for LEO orbits and the South Atlantic Anomaly.
• MeV electrons penetrate, causing deep dielectric
discharges, such as the Telstar 401 satellite loss.
• The Geosynchronous orbit uniquely lies on the edge
of a steep spatial and spectral gradient. Thus GEO
poses a wildly variable MeV electron environment,
which is nearly impossible to predict, both in
principle and in practice.
• Yet of all orbits, this is THE most crowded spot.
ORBE (McIlwain 1966)
McIlwain 1966
Details of Injection (#1&5)
• They have a 24-48 hour typical rise time from a SW
disturbance (shock, Dst storm, etc.) But can be as
short as 8 hr (Jan 97) or as long as 72 hours.
• The intensity roughly follows a solar cycle
dependence but can vary by 3 orders of magnitude
• The spectral hardness generally increases during the
storm, but exhibits a “knee” that whose cutoff
strongly depends on the unpredictable magnitude
• Best correlation with Vsw (70%) but Bz or n_sw
ruins it. Neither mechanical nor electrical SW driver
Koons & Gorney Prediction Filter
The Prediction Paradox
• The storms we want to predict, are the BIG ones,
but our best statistics are the little storms. Statistical
precursors, neural nets, linear filters ALWAYS do
better on the little storms, not the killer storms.
• We will never achieve reasonable prediction until
we understand how/where/why BIG storms occur.
Everything else is gambling.
• But the statistics have not led to better physics. Why
is Vsw the best correlator? Especially since Dst is
better correlated to Ey and AE to Bz. Why is Kp the
best correlated ground-based index?
The Argument
1) Electrons, not protons are injected.
2) Radial gradients point to an external source that is
NOT the solarwind, NOR the dipole trapped belts.
3) Risetimes are too slow (2 days) to be 1st order
acceleration, more likely 2nd order stochastic,
ordered by energy dependent exponential risetimes.
4) Neither AE nor Dst correlate as well as Kp. Global
disturbances work better than tail or RC.
5) MLT enhancements begin at noon.
6) Low altitude data are consistent with simultaneous
inward radial transport & PA scattering
Problems with Cusp Acceleration
• We said that Cusp acceleration was continuous,
given continuous injection. How then do we explain
the abrupt injections observed in storms &
McIlwain’s data?
• The solution is topology. Abrupt changes in
topology of the cusp also cause abrupt changes in
output. The key is that the cusp is a POSITIVE
feedback amplifier, and can be driven to the rails by
appropriate input.
• The plasma trapped in the cusp also contributes to
the magnetic field topologymaking it stronger!
Diamagnetic Effect of Cold Plasma
Cusp Diamagnetic Cavities
Energetic Energy Dispersed Events
Model Predictions
• MeV electrons are born in CDCs
• Not solar wind Vsw but DVsw drives MeV e
• Conditions amenable to driving plasma in the cusp
are the predictors for MeV e events:
– Cusp tilted toward the Sun. Solstices over equinox.
– High momentum solar wind.
– Bz northward during impact. Turbulence afterward.
• The longer the CDC lasts, the higher the “knee”, and
the harder the spectrum
• MeV-Dst correlations are also due to DVsw
Scaling Laws
•
•
•
•
•
Brad ~ Bsurface= B0
Bcusp ~ B0/Rstag3
Erad= 5 MeV for Earth
Ecusp ~ v2perp~ (Bcuspr)2 ~ [(B0/Rstag3)Rstag]
m = E/B is constant
Erad-planet~(Rstag-Earth/Rstag-planet)(B0-planet/B0-Earth)2Erad-Earth
Scaled Radiation Belts
Planet
Mercury
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
R STAG
1.4
10.4
1.25
65
20
20
25
B0 (nT)
330
31,000
<6
430,000
21,000
23,000
14,000
ERAD
4 keV
5 MeV
< 1.5 eV
150 MeV
1.2 MeV
1.4 MeV
0.42 MeV
More Predictions
• The unpredictability arises from a strong nonlinearity in the trapping ability of the cusp. Small
misalignment of cusp & no CDC is formed.
• Note that high Vsw usually means high DVsw, and
simultaneously, higher seed energies for 2nd order
Fermi acceleration. Thus a synergistic correlation.
• Magnetic clouds are also geoeffective, but with very
different properties than high speed streams (Jan 97)
• CDC probably “evaporate” releasing lowest energy
last, causing Time-Energy Dispersions.
• Positive feedback means many inputs=same output
Problem #2: Source of GCR
GCR Spectra
Properties of GCR
• Energy density of GCR = 1 eV/cc (~6 @ galact. ctr)
• Energy density of Interstellar Medium components:
• GCR have equivalent energy to all other ISM stuff.
Are GCR from Supernovae?
• Power output of Supernovae shock ~1051/30yr = 1035W, of
which and estimated 15% show up in GCR, or 2e34 Watts.
• Lifetime of GCR ~ 1015s. (from 10B spallation)
• Energy Density * Volume /Time = 1eV/cc*1069cc/1015s
= 3x1035Watts (and it only gets worse if you use the numbers
in the galactic center) giving a ratio: SN/GCR = 0.1 !
Another calculation: Energy Density * velocity * area =
luminosity  1eV/cc * 3e10cm/s *5e45cm2 = 1e35 Watts
• Even if the entire energy of a supernova went into GCR, and
as we argued earlier, acceleration is a very inefficient
process, we would still have an energy budget problem!
• (As some wag put it, SN are already highly oversubscribed,
everyone already invokes it for their energy source)
• Where is the energy for GCR coming from?
Low energy
nuclei
composition
Some more peculiar coincidences
•
•
•
•
•
•
Energy density of starlight
= 0.3 eV/cc
Energy density of ISM
= ~1 eV/cc
Energy density of interstellar B-fields = 0.2 ev/cc
Cosmic Background Radiation
= 0.3 eV/cc
Nuclei 98%, electrons 2%
Everyone calls these “coincidences”, but perhaps
there is a theory that links them all together. My
contention is that quadrupole cusp acceleration is
just such a proto-theory.
Quadrupole Cusps Pressure Balance
• Dimensional analysis: Energy/Vol  Force/Area =
Pressure. Thus mechanisms that equalize pressure
will also equalize energy density.
• In a galaxy with a dipole magnetic field embedded
in a flowing plasma, the cusp topology (and strength
of the magnetic field) is affected by the ram
pressure. Thus we can write an equilibrium:
PGCR + Pmag + Pstarlight = Pram_H + PCMB
• Assuming that equipartition has balanced the IGM
Pram_H = PCMB
• Thus we explain all these “coincidences” as a
pressure equilibrium in the quadrupole cusp
Energy Sources for GCR
• Where does the energy come from? Supernovae of
course! Seriously, shock waves travelling out of the
galactic disk transmit energy to the cusp & compress
it, just as much as turbulence in the intergalactic
medium (IGM). The cusp is a low-Q object, energy
(waves) goes in, and doesn’t come out.
• The advantage over Fermi-acceleration at SN?
Continuous acceleration, multiple energy sources,
identifiable rigidity properties. And the clincher…
• A natural explanation of the “knee”. At low-E,
protons have the smaller rigidity, at high E, (due to
gamma) Fe has the smaller rigidity. So they cross.
Future Work
• Clearly this proto-theory is crying out for some
modelling.
• Anomalous cosmic rays, coming from the
heliopause shock, are also thought to be “shock
accelerated”. If the GCR theory is correct, it should
also apply to ACR. There is a great deal more data
on ACR, which should make it a critical test case for
the validity of the model.
• The Earth’s cusp is far more dynamic than either the
heliosphere or galactosphere. Data from Earth
satellites is most extensive. Work is needed to make
a dynamic & predictive model of ORBE.
Conclusion
• The Hamiltonian Dynamics of Quadrupole Traps
show interesting features that permit rapid stochastic
diffusion.
• Quadrupole traps have the right properties to make
highly efficient accelerators at many length scales.
• The signatures of such accelerators--rigidity cutoffs,
pressure balance, positive feedback--may resolve
many outstanding problems in space & astrophysics.
Soli Deo Gloria